For me it was cause the bulb had gone, only one side has a bulb unless you wire across to the other cluster, can't remember which side is fog.
Other than that check the power to the switch does it illuminate, does the little orange bit at the bottom light (note that this won't light if the bulbs blown but the switch will still illuminate) if you work out it's your switch I think I've got a spare, I'll check later.
